1. Field of the Invention
This invention relates to an electrophotographic printing device using a photosensitive unit which is required to be periodically replaced.
2. Description of the Related Art
In general, a laser printer has a photosensitive unit. The service life of the photosensitive unit terminates when a number of printing operations are effected and it becomes necessary to replace the photosensitive unit. With the laser printer, an electrostatic latent image is formed on the photosensitive surface of the photosensitive unit and then toner is supplied to the photosensitive surface. Part of the toner is attached to the photosensitive surface according to the electrostatic latent image and the remaining toner is fed as spent or used toner into a used-toner box.
The amount of used toner increases as the printing operation is repeatedly effected. Conventionally, the capacity of the used-toner box is so set that the used-toner box will be filled with the used toner when printing operations of a number corresponding to the replacement time of the photosensitive unit are effected, and the service life of the photosensitive unit is checked according to an output signal generated from a used-toner sensor mounted on the used-toner box when it has detected that the used-toner box is filled with the used toner.
However, if the replacement time of the photosensitive unit is determined according to the amount of the used toner stored in the used-toner box, it is impossible to detect the optimum replacement time of the photosensitive unit since the amount of used toner obtained in each printing operation may vary according to the printing operation. The used-toner sensor is relatively expensive and it is difficult to lower the manufacturing cost.